Fix issue where "phabricator" disqus account is used by default and not overridable

Summary: Currently setting "disqus.shortname" via config isn't actually used in the instantiation of disqus. This fix uses the shortname configured.

Test Plan: Open a phame post with disqus enabled and a shortname other than "phabricator" specified.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6310